The present invention relates to a method for providing a coating layer of crystalline silicon carbide on the surface of a substrate, such as a ceramic plate used as a substrate in the preparation of various kinds of electronic devices. In particular, the invention relates to a method for the prepartion of a carbon fiber coated with siicon carbide to form a sheath-and-core structure.
As is well known, silicon carbide of high purity is a material having excellent properties such as high heat resistance, high resistance against oxidation or attack of chemicals, good heat conductivity and others. High-purity silicon carbide is also semiconductive electrically so that there are attempts in the field of electronics technology to provide a coating layer of silicon carbide on to the surface of various kinds of electronic materials including semiconductor substrates as well as jigs or other articles used in the processing of semiconductor devices by utilizing the above mentioned unique properties thereof. It is also desirable that various kinds of sealing materials and heat-conductive members are provided with a coating layer resistant to various influences from the environment as may be obtained by providing a coating layer of silicon carbide.
Turning now to carbon fibers manufactured by the infusibilization and carbonization of rayon, polyacrylonitrile and other organic fibers or filaments of lignin, pitch and other bituminous materials, they are widely used in various fields as a promising material with high mechanical strength and heat resistance although carbon fibers are unavoidably oxidized in an oxidizing atmosphere at high temperatures. For example, carbon fibers are used as a reinforcing material in various types of so-called composite materials with a synthetic resin or a metal as the matrix material. A problem in such an application of carbon fibers is the relatively poor wettability or affinity of the surface of carbon fibers with the molten resin or metal. Furthermore, certain metals react with carbon fibers at high temperatures to form a metal carbide so that the application of carbon fibers as a reinforcing material of fiber-reinforced metals is considerably limited.
Silicon carbide can also be obtained in a fibrous form free from the above mentioned problems of carbon fibers as a reinforcing material of synthetic resins and metals. That is, silicon carbide is highly resistant against oxidation and has a good wettability or affinity with molten resins and metals but has no reactivity with molten metals even at extremely high temperatures. Unfortunately, silicon carbide fibers are very expensive because they are manufactured in a very complicated process including preparation of a specific organosilicon polymer which is then spun into filaments followed by infusibilization and firing thereof into silicon carbide.
Accordingly, it would be an easy idea that the defects of the carbon fibers as a reinforcing material of composite materials may be overcome by providing a coating layer of silicon carbide on the surface of carbon fibers.
The prior art processes for providing a coating layer of silicon carbide on to the surface of a solid as a substrate are performed according to several different principles. For example, (1) silicon carbide is sublimated at an extremely high temperature of 2000.degree. C. or above and the vapor of silicon carbide is deposited on the substrate surface to form a coating layer (see, for example, Japanese Patent Publication No. 41-9332), (2) a gaseous mixture of a chlorine-containing silane compound represented by the general formula (CH.sub.3).sub.a SiCl.sub.4-a, in which a is 0, 1, 2 or 3, and a hydrocarbon compound such as methane is pyrolyzed on the surface of the substrate heated at a high temperature to deposit silicon carbide formed in situ (see, for example, Japanese Patent Publication No. 44-18575), (3) a gaseous mixture of silane SiH.sub.4 and a hydrocarbon compound is pyrolyzed on the substrate surface heated at a high temperature to deposit silicon carbide formed in situ (see, for example, British Pat. No. 1,039,748), and ( 4) a powdery mixture of silicon dioxide or elementary silicon and carbon is heated at 1500.degree. C. or higher in contact with the substrate surface so that the silicon carbide is deposited on the substrate surface as it is formed by the reaction (see, for example, Japanese Patent Kokai No. 52-42365). Limiting the subject mater to the method for providing a coating layer of silicon carbide on to the surface of carbon fibers, (5) a method is known in which a gaseous chlorine-containing silane compound such as silicon tetrachloride and trichlorosilane is pyrolyzed on the surface of carbon fibers to deposit elementary silicon followed by heating of the thus silicon-coated carbon fibers at a high temperature to effect the reaction between carbon and silicon into silicon carbide so that a filament having a sheath-and-core structure is obtained (see, for example, Japanese Patent Kokai No. 50-38700).
Unfortunately, each of the above described prior art methods is not quite satisfactory in one or other respects when industrial practice of the method is intended. To explain it, the first method is subject to the limitation of the kind of the substrate material because the method is performed at an extremely high temperature of 2000.degree. C. or above so that the substrate material also naturally must withstand this high temperature. The second method has a problem, in addition to the considerably high temperature for the pyrolysis, that the starting chlorine-containing silane compound is readily hydrolyzed when in contact with atmospheric moisture requiring utmost carefulness in handling and the hydrogen chloride formed by the reaction as a byproduct is a hardly disposable material of nuisance sometimes to cause environmental pollution. The third method is, although it is advantageous in obtaining a coating layer of crystalline silicon carbide at a relatively low temperature, disadvantageous because of the great difference in the temperatures of pyrolysis and the reaction velocities between the silane and the hydrocarbon compound requiring delicate control of the concentrations of them or addition of hydrogen chloride to the reaction system since otherwise the resultant coating layer of silicon carbide has poor uniformity. The fourth method is also subject to the limitation of the applicable substrate materials due to the high reaction temperature of 1500.degree. C. or higher. Further, the fifth method applied to carbon fibers is industrially unfeasible because of the complicacy of the process in addition to the high temperature conditions required for the reaction.
Thus, it would be a problem of great industrial importance to develop a novel and improved method for providing a coating layer of silicon carbide on to the surface of substrates including carbon fibers.